This application is based on Application No. 2000-293606 filed in Japan, contents of which is hereby incorporated by reference.
1. Field of the Invention
The present invention relates to an image forming apparatus for forming a full color image by superimposing different color images. More particularly, the present invention relates to an image forming apparatus capable of reducing an occurrence of color registration error and maintaining uniform coloring among pages.
2. Description of Related Art
In an image forming apparatus for forming a full color image by superimposing different color images, it is required to precisely superimpose images on one another. Otherwise, color registration error occurs and an image quality is degraded. Such color registration error occurs because an image transfer position of a reference color and that of other colors are relatively shifted one another due to a speed change or the like of a transfer belt.
In order to prevent such color registration error, the following registration correction is generally carried out. That is, a plurality of registration patterns are formed on a transfer belt prior to forming an image. These patterns are repeatedly sampled at a predetermined timing by a registration detecting sensor. Further, a registration quantity of other colors (for example, cyan, magenta, or yellow) relevant to a reference color (for example, black) is detected from the thus obtained sampling data, and this detected quantity is divided by the sampling count, thereby calculating an average registration quantity. Then, registration correction (color registration error correction) is carried out based on this average registration quantity.
Further, a technique for improving precision of this registration correction is disclosed in Japanese Laid-open Patent Publication No. 10-148992. In this technique, during a non-image formation period, a registration pattern is formed over the entire periphery of a transfer belt. Next, these patterns are repeatedly sampled at a predetermined timing by a registration detecting sensor, registration quantity is detected at each sampling timing, and a typical value of the registration quantity is calculated. Then, difference data between the registration quantity at each sampling timing and the typical value is calculated. On the other hand, during an image formation period, registration patterns are first formed between image forming regions of a transfer belt. Next, these registration patterns are repeatedly sampled at a predetermined timing by a registration detecting sensor, the registration quantity at each sampling timing is detected, and the respective registration quantities are compensated for by the corresponding difference data. Then, the compensated data is employed to calculate the typical value of the registration quantity.
In this manner, during an image formation period, a change in registration quantity for each sampling timing is offset by the corresponding difference data. Thus, during an image forming period as well, the typical value of the registration quantity can be calculated at a level equal to that during a non-image formation period.
However, in the above described conventional technique, there has been a problem that an occurrence of color registration error cannot be completely reduced. This is because a deviation occurs between the typical value of the registration quantity employed for registration correction (average value of total sampling values) and a correction quantity actually required for registration correction. That is, a position change on a transfer belt occurs periodically, and thus, a correction quantity required for each image forming region changes depending on a position of an image forming region on the transfer belt. Therefore, in a region in which the position change quantity greatly differs from the typical value of the registration quantity, if registration correction is carried out based on the typical value of the registration quantity at the entire periphery of the transfer belt (average value of total sampling values), color registration error becomes significant. In addition, in the case where images are formed by a plurality of pages, the required correction quantities differ with each page. Thus, color registration error that occurs with each page changes. Therefore, there has been a problem that the coloring of each page changes.
The present invention has been made in order to solve the foregoing problem. It is an object of the present invention to provide an image forming apparatus capable of preventing an occurrence of color registration error and maintaining uniform coloring among pages by determining a typical value of an optimal registration quantity for each image forming region.
According to one aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image forming units for forming images with different colors; an image carrier for carrying a multi-color image produced by superimposing images formed at each of the image forming units sequentially; a position detecting sensor for detecting a reference position provided at the image carrier; a position change quantity calculator for calculating position change quantity of other colors against a reference color; a storage device for storing position change quantity calculated by the position change quantity calculator; an image forming start position calculator for calculating an image forming start position based on a detection signal detected by the position detecting sensor; an image forming region calculator for calculating a position of an image forming region on the image carrier based on an image forming start position calculated by the image forming start position calculator and a length of an image formed by each of the image forming units; a typical value calculator for reading out position change quantity at a predetermined position of the image carrier that corresponds to an image forming region calculated by the image forming region calculator from the storage device, and calculating a typical value of the position change quantity readout; and a controller for controlling operation of the plurality of image forming units based on the typical value for each image forming region calculated by the typical value calculator. The xe2x80x9cposition change quantityxe2x80x9d means a quantity of position error of other color relevant to a reference color caused by speed change or the like of an image carrier.
This image forming apparatus comprises a plurality of image forming units for forming an image with different colors, a controller for controlling operation of the plurality of the image forming units; and an image carrier for carrying a multi-color image produced by superimposing images formed at each of the image forming units sequentially. Thereby, a multi-color image can be formed. Here, when sequentially superimposed multi-color images are carried, they may be carried on the image carrier directly or may be carried indirectly via a recording medium. That is, the present invention is applicable to an image forming apparatus in accordance with an ink jet process or a silver salt process and the like as well as an image forming apparatus in accordance with an electrophotographic process. In case of electrophotographic process or ink jet process, plain paper is primarily used as a recording medium. In case of silver salt process, photosensitive paper is primarily used as a recording medium.
In order to form a multi-color image by thus superimposing different images, the image forming apparatus according to the present invention carries out the following control so that the images of colors can be precisely superimposed on each other. First, a reference position provided at an image carrier is detected by a position detecting sensor. Then, during a non-image formation period, position change quantity of other colors relevant to the reference color is calculated by the position change quantity calculator. The position change quantity calculated by the position change quantity calculator is stored in the storage device. The non-image formation period includes a state in which an image is not formed and a state immediately after the image forming apparatus is powered ON.
During an image formation, an image forming start position is first calculated by an image forming start calculator based on a detection signal detected by a position detecting sensor. Next, an image forming region calculator calculates a position of an image forming region on an image carrier based on the image forming start position calculated by the image forming start position calculator and a length of an image formed by each of the image forming units.
Further, by a typical value calculator, the position change quantity at a predetermined position of the image carrier that corresponds to the image forming region calculated by the image forming region calculator is read out from the storage device, and the typical value of these position change quantities is calculated. Then, operation of each image forming means is controlled by a controller based on the typical value for each image forming region calculated by the typical value calculator. That is, registration correction is executed based on the optimal typical value for each image forming region. In this manner, an occurrence of color registration error can be reliably prevented. In addition, even when images are formed by a plurality of pages, registration correction for each page is done based on the optimal typical value. Thus, uniform coloring among pages can be maintained.
According to another aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image forming units for forming images with different colors; an image carrier for carrying a multi-color image produced by superimposing images formed at each of the image forming units sequentially; a position detecting sensor for detecting a reference position provided on the image carrier every traveling cycle of the image carrier; a position change quantity calculator for calculating position change quantity of other colors against a reference color at a respective one of a plurality of predetermined positions in the image carrier determined based on a detection signal detected by the position detecting sensor; a storage device for storing the position change quantity calculated by the position change quantity calculator; an image forming start position calculator for calculating an image forming start position based on the detection signal detected by the position detecting sensor; an image forming region calculator for calculating a position of an image forming region on the image carrier based on the start position calculated by the image forming start position calculator and a length of an image formed by each of the image forming units; a typical value calculator for reading out position change quantity at a predetermined position of the image carrier that corresponds to the image forming region calculated by the image forming region calculator from the storage device, and calculating the typical value of the position change quantity readout; and a controller for controlling image write timing of each image forming unit based on the typical value of each image forming region calculated by the typical value calculator.
This image forming apparatus as well carries out control that is basically similar to the above described image forming apparatus. Therefore, operation of each image forming unit, i.e., image write timing, is finally controlled by a controller based on a typical value of each image forming region calculated by the typical value calculator. Here, the position detecting sensor detects a reference position every traveling cycle of the image carrier. In addition, by the position change quantity calculator, in a plurality of predetermined positions of the image carrier determined based on the detection signal detected by the position detecting sensor, the position change quantity of other colors against the reference color at a respective one of these predetermined positions is calculated. Based on these facts, registration correction based on an optimal typical value can be reliably executed with respect to each image forming region. Therefore, an occurrence of color registration error can be prevented precisely, and uniform coloring among pages is maintained.
According to still another aspect of the present invention, there is provided an image forming apparatus comprising: a plurality of image forming units; an image carrier for carrying an image produced by superimposing images formed at the plurality of image forming units; a detection device for detecting a position of an image forming region on the image carrier; a calculating device for calculating a typical value of position change quantity of other colors against a reference color in an image forming region detected by the detection device; and a controller for controlling operation of the plurality of image forming units based on a typical value calculated by the calculating device.
In this image forming apparatus, images are formed by a plurality of image forming units, and are carried by the image carrier with being superimposed on one another. By doing this, the detection device detects a position of image forming region on the image carrier. Subsequently, the typical value of the position change quantity of other colors against a reference color in the image forming region detected by the detection device is calculated by the calculating device. Then, operation of a plurality of image forming units is controlled by the controller based on the typical value calculated by the calculating device. Namely, registration correction is executed based on optimal typical value for each image forming region. In this manner, an occurrence of color registration error can be reliably prevented. In addition, when images are formed by a plurality of pages, registration correction is done based on the optimal typical value for each page. Thus, uniform coloring among pages can be maintained.